Interstellar visitor crosses the Martian sky
As October 2025 unfolded, astronomers faced a familiar puzzle: how do you observe something that’s on the opposite side of the Sun? The answer, in part, lay with the many rovers and spacecraft stationed on Mars. From 1 to 7 October, Europe’s ExoMars Trace Gas Orbiter (TGO) and Mars Express turned their cameras toward 3I/ATLAS, the rare interstellar comet crossing through our cosmic neighborhood, to capture whatever clues they could as it passed close to Mars.
3I/ATLAS, discovered in July 2025, is not a comet born in our Solar System. It hails from another star, making a fleeting appearance as it sails past the Sun rather than orbiting it. Its visit offered a rare opportunity to study interstellar material up close, and the Mars-orbiting spacecraft were well-placed to contribute.
What the Mars missions saw
ExoMars TGO used its Color and Stereo Surface Imaging System (CaSSIS) to image 3I/ATLAS. The result is a white dot streaking across the Martian sky in an animation stitched from individual frames. The dot represents the comet’s nucleus and its surrounding coma, though the nucleus itself remains indistinguishable in these early frames. The coma, a cloud of gas and ice sublimating as the comet warms, spans several thousand kilometers, a testament to the dramatic activity as 3I/ATLAS nears the Sun.
Mars Express contributed additional context with its cameras and spectrometers. However, imaging a distant interstellar object is a tall order for instruments designed to photograph Mars’ surface a few thousand kilometers away. Mars Express’ exposure times were short (about 0.5 seconds), compared with TGO’s 5 seconds, limiting the clarity of any direct nucleus view. Still, scientists are hopeful that deeper analysis could pull meaningful signals from the data.
What the data tell us about the comet
Even with limited resolution, researchers were able to use spectroscopy to probe 3I/ATLAS’s composition. The light scattered or emitted by the comet carries fingerprints of materials like carbon dioxide, water ice, water vapor, and carbon monoxide. These signatures can reveal how the comet formed, how it has evolved, and how its surface reacts as it approaches the Sun.
Early analyses also suggested that 3I/ATLAS could be one of the oldest comets observed—perhaps seven billion years old—making it a fossil from the early days of the galaxy. Its interstellar origin adds another layer of intrigue, providing a rare opportunity to compare material from another star system with that collected from comets within our own solar neighborhood.
What’s next for the interstellar visitor
The international team isn’t finished with 3I/ATLAS. Scientists will continue to comb through the Mars data in the coming weeks and months, refining their understanding of the comet’s composition, activity, and behavior as it continues to drift toward the Sun. The spectral measurements from OMEGA, SPICAM (on Mars Express), and NOMAD (on TGO) offer the best chance to piece together a robust chemical profile of the visitor, even if a direct, high-resolution image of the nucleus remains elusive.
ESA’s ongoing mission plan: Jupiter’s turn
ESA isn’t done with 3I/ATLAS yet. In November 2025, the Jupiter Icy Moons Explorer (Juice) will study the comet as it passes in the vicinity, albeit from a greater distance. Juice will observe 3I/ATLAS at a different stage of its activity, potentially capturing the tail’s evolution as it is heated by the Sun. Although Juice’ observations will not arrive until February 2026, they will augment the Mars data with a broader perspective on how interstellar material behaves in different solar environments.
Bottom line for skywatchers and scientists alike
The episode demonstrates how even a mission not designed for deep-space comets can contribute to frontier science. The observations from Mars’ orbiters, combined with data from Earth-based telescopes and upcoming measurements from Juice, offer a unique, multi-perspective view of a visitor from another star. As 3I/ATLAS continues its solar journey, every photon—whether captured by a high-powered telescope on Earth or a low-exposure frame from CaSSIS—adds to our understanding of the cosmos and our place within it.
“This was a very challenging observation for the instrument,” notes CaSSIS Principal Investigator Nick Thomas, highlighting the difficulty of seeing something 10,000 to 100,000 times fainter than typical targets. Yet the team’s persistence pays off, expanding our knowledge of interstellar materials and helping humanity prepare for future, even more elusive visitors.
